Gas turbine engine test cells are well known in the art and are used for testing and measuring performance of newly designed or recently overhauled gas turbine engines. An example of a typical out-of-airframe engine test cell is shown in FIG. 1. Such test cells commonly include an inlet stack 10, a test section 20 housing the gas turbine engine 30 to be tested, an augmentor tube 40 and an exhaust stack 50.
As depicted in FIG. 1, the inlet stack 10 of the conventional test cell typically includes intake splitters 12 (for acoustic treatment) and turning vanes 14. Separating the intake stack from the test section 20 is typically a flow screen 16 with a roll-up door 18.
As further depicted in FIG. 1, the test section 20 typically comprises a thrust frame 22 and monorail system 24 for mounting the gas turbine engine 30. In this conventional test cell, exhaust gases from the gas turbine engine are exhausted into an ejector comprising an augmentor tube 40, diffuser 41 and exhaust basket 42. The augmentor tube may be enclosed within a chamber known as an augmentor enclosure 51. The augmentor tube detunes the flow of exhaust gas. Exhaust gases are then emitted into the exhaust stack 50 from the exhaust basket 42.
During operation, the engine draws air into the test section through the inlet stack and exhausts flow out of the test section and into a large diameter tubular structure commonly referred to as an augmentor or augmentor tube which is connected to a diffuser and exhaust basket. Flow is directed from the augmentor, diffuser and basket into the base of a vertically oriented exhaust stack which exhausts to atmosphere.
Gas turbine engine test cells of the type described above are designed to function as a pump to maintain sufficient air flow through the test section to provide proper aerodynamic simulation and flow rates while minimizing noise and vibration to the surrounding environment.
The problem of noise treatment for a gas turbine engine test cell generally falls into two categories defined by two distinct regions of the sound frequency spectrum: the audible range which is generally acknowledged to extend from approximately 20 Hz through about 20 kHz and the inaudible (“infrasound”) range occurring at relatively low frequencies from a few Hz to about 50 Hz. Both frequency ranges present distinct problems and concerns and thus require different solutions.
The audible part of the sound spectrum generated by the test cell corresponds to wavelengths which are small relative to the characteristic dimensions of the test cell and results principally from sound waves propagating from the test engine, through the system and out into the environment. The accepted solutions for dealing with sound waves in the audible frequencies are straightforward and commonly involve the use of acoustic baffles in the inlet stack and exhaust stack as well as the use of acoustic pillows. It has been found that such baffles and pillows are able to dissipate sound waves in the audible frequency range to an acceptably low level.
Infrasound, however, occurs at wavelengths that are large relative to the characteristic dimensions of the test cell and thus result in what are considered standing wave patterns rather than propagating noise. Although infrasound is not audible, and thus does not present readily detectable concerns to the surrounding population, the relatively large wavelengths of infrasound present its own unique set of problems and concerns. For example, large buildings and other structures or parts thereof will vibrate or tend to vibrate at certain natural frequencies in the infrasound range. The concern is that repeated exposure to infrasound frequencies over an extended period of time could result in structural problems. The concern exists not only with respect to buildings and other structures in existence at the time the test cell is installed, but buildings and other structures which may be erected years later as the community and surrounding businesses develop. Furthermore, there are health concerns with respect to these vibrations.
Although a variety of sound attenuation techniques are known in the art, infrasound remains problematic with augmentor tubes. Hard-to-treat low-frequency noise is known to develop inside the augmentor tube as the jet turbulent eddies grow inside the tube. A further issue with augmentor tubes is ensuring that the augmentor tube is able to pump the minimum amount required to meet the total air mass flow rate required for the test chamber.
A further challenge with respect to gas turbine engine test cells is their substantial size. Due to the length of the augmentor tubes, the ejector systems typically range from 10 to 60 meters long (30 to 190 ft). A long ejector system is required for adequate air flow mixing and test cell ventilation (test cell bypass). Conventional test cells thus typically occupy a large footprint, which means that it is often difficult and expensive to acquire the land to build a new test cell.
Yet a further problem that arises with conventional test cells is that an existing facility cannot easily be enlarged to accommodate testing of larger and more powerful engines. To enlarge an existing facility requires that the building be renovated or extended to accommodate a longer ejector system. This may only be possible where adjoining lands are available.
In view of these various issues and challenges with the prior art technology, there remains a definite need in the industry for an improved gas turbine engine test cell and an improved method for testing gas turbine engines.